Calciners and particularly indirectly-heated rotatable calciners are utilized for heat-treating and drying of materials at temperatures higher than those normally employed in steam-heating apparatus. For example, indirectly-heated calciners having a rotating cylindrical retort are frequently employed to reduce mineral oxides to low oxides, dry and remove sulfur from cobalt, copper and nickel powders, reduce metal oxides, and numerous other similar applications wherein relatively high temperatures are desirable.
In the known types of calciners employing a rotating cylindrical retort, it is a common practice to feed the materal for processing into one end of the retort and collect the processed material from an opposite or discharge end. Also, a series of flights attached to the inner surface of the rotatable retort or a "scraper" chain may be utilized to control the progress of the material as it is processed through the rotating cylindrical retort.
Of the known forms of material feeding apparatus for such rotating calciners, one of the more common is a two-turn spiral structure which uniformly tapers from from a smaller to a larger height as the material progresses to the rotating calciners. Obviously, added turns may be provided prior to the above-mentioned two-turns spiral structure should it be necessary or desirable to increase the distance over which the material is transported.
Although the above-mentioned uniformly tapered material feeding spiral structure has been and still is used with varying degrees of success, it has been found that these are applications wherein such structures do leave something to be desired. For example, it has been found that the rate of material flow and consisting of the feeding of material to the cylindrical retort is less than desired when structures of the above-described configuration are employed. Also, it has been found that the material already in the cylindrical retort had a tendency to feed back into the feeding spiral on structures of the above-described configuration.